Electron tube



April 16, 1940. w. smz ETAL 2,197,652

ELECTRON TUBE" Filed March so, 1938 Fig. I.

Inventors: Willi Statz, Friedrich Malsch,

by W 6 T i -gr Attorhy- Patented 16, 1940 P TE T'OFFI E ELECTRON TUBE Willi Statz, Brieselang, Osthavelland, and-Friedrich Malsch, Berlin-Charlottenburg, Germany, assignors to General Electric Company, a corporation of New York Application March 30, 1938; Serial No. 198,942 I In Germany April' 8, 1937 i 1 Claim. (c1. 250-) 1 The present invention relatesto electron tubes and more particularly to beam tubes which employ secondary emission. The primary object of the invention is to provide a tube of the type referred to and in which an improved control is exercised over the number of secondary electrons produced at the target and the number of secondaries which actually reach the anode. In brief, our invention contemplates an'improved electron focusingeffect for controlling the production of the secondaries and an improved arrangement of electrodes for controlling the number of secondaries which reach the anode.

The invention will be better understood when considered in the light of the following specification and the accompanying drawing in which Fig. l is a diagrammatic view of a tube provided with a. variable focusing arrangement, and Fig. 2 shows a tube in which the number of secondaries reaching the anode is controlled by the relative position of the secondary emission element with respect to the anode.

In Fig. I, numeral I designates a highly'evacuated envelope which contains asource 'of pri mary electrons, preferably an indirectly heated cathode, indicated diagrammatically by the cirole-2; Positioned about three sides of the cathode, there is a rectangular shield member 3, open at one side and charged to such a potential as to produce an initial focusingof the electron stream. At the opposite end of the tube there is mounted an electrode structure for receiving the beam.- Secondary electrons are generated at various surfaces on this structure when the latter is struck by the primary electrons which move through the tube at high speed. A coil indicated at 5 is mounted outside of the envelope between the said electrode structure and'the cathode for thepurpose of producing a magnetic field which changes the focus of the beam, depending upon the current passing through the coil. When the beam is constricted to a relatively small size, it strikes a circular plate member or collecting electrode 6 which is provided with a cylindrical flange 1. Any secondary electrons generated at the plate 6 are intercepted by the cylindrical member I.

At the same end of the tube as the plate memher 6, there is provided a frustro-conical metal member 8 which surrounds the member 6 and has a central opening 9 at its apical portion sufficiently large for the electron beam freely to pass through when the beam is constricted to a relatively small size by the magnetic focusing field. The outer surface of the conically shaped member 8 is preferably coated with a material such as alkaline earth 'oxide'which readily gener'ates secondary electrons when struck by a fast moving beam of primaryelectrons.- The beam is accelerated through the tube by suitably applied potentials. It is apparent that the current through the coil 5 may be so controlled as to constrictith'e electron beam to a small size such as to forma focalspot entirely within the cylinder 1, in which case substantially all of the generated secondaries are intercepted'by the cylinder. This positionof the beam is indicated roughly by the dot and dash lines 10. On the "other-hand; the current through the coil 5 may be such as to cause'or permit the electron beam to spread out or distribute itself, asindicated by the dotted lines I Land to strike for the most part the outer surface of themember 8. The number of secondaries generated at the surface 8 may be'greatly fieldtow'ard an anode l2 which conveniently may t'akethe form of acone sh'apedmernber having alarg'e opening. thecurrent through the coil ,5 is varied in accordance with the variations of a contrel potentiafde'rived for example from a source 511, thefocus or the beam is also varied and the number of secondaries produced at the element 8 is correspondingly changed. There is a similar change in the number of secondaries reaching the anode, thus giving rise to variations in the anode current. These current variations I therefore representin amplified form the changes of current passing through the coil 5 and can be usedin any suitable and well-known manner. Instead of providing the cylinder 1 for intercepting secondary electrons incidentally generated at the plate 6 and instead of providing the conic'ally shaped anode 8, we may employ the member 6 as the main secondary emission element for controlling the anode current. In this case the plate 6 may be provided with two or more annular surfaces of different secondary emission properties obtained in any well-known and suitable manner so that as the focus of the electron beam is changed by the magnetic field, secondary electrons are generated at the surface of the plate 5 in a variable manner, depending upon which of the annular surfaces is struck by the electron beam. It is apparent that the current through the coil may be such as to change the distance of the focal point of the beam with respect to thecathode so that the focal point may be posijtioned either in front oil the plate 6 or beyond the plate 6, in which case the size of thefocal spot is also changed and the secondary emission from the various annular surfaces controlled in like manner. The secondary electrons obtained in this manner may be caused by the application of suitable potentials to flow directly to the anode I2 and cause a change in the anode current. Instead of providing a magnetic field for controlling the size of "the electron beam, we may utilize an electrostatic form of control for this purpose. For example, a metal cylinder may be mounted within the envelope charged to a variable potential, or any other suitable form of electrostatic focusing means may be employed.

Instead of employing an electric focusing arrangement for controlling the number of secondary electrons which reach the anode, we may use a beam deflection structure for this purpose. Thus in Fig. 2, the anode l3, which may comprise a fiat sheet of metal, is positioned in adjacent parallel relation with respect to one of the plates M of a multi-sectioned secondary emitter element. The plates M which preferably are mounted normal to one another, as shown in the drawing, may be coated on the side facing the cathode with any well-known and suitable material which pro-' fusely emits secondary electrons. The cathode l5 may be of the indirectly heated type as in the case of the corresponding element shown in Fig. 1.

which may be charged to any suitable'positive potential. It willbe noted that this cylinder extends considerably beyond the deflecting plates H5 in order, effectively to exercise the necessary focusing control.

The negative potential on one or both. of the plates l5 maybe controlled in any well-known and suitable manner, thus making one of the plates 'morenegative with respect to the cathode than the" other plate. Under these conditions,

the electron beam which is roughly indicated by' the dotted lines l8 will swing across the tube the anode and the secondary emission element may be obtained. The changes in the anode current brought about by the deflection of the electron beam IB may be utilized in any suitable and Well-known external circuit. These current changes will correspond in a general way to the changes in the bias potentials applied to the defleeting elements I6.

' What we claim as new and desire to secure by Letters Patent of the United States, is:

An electric discharge device comprising an electron source for producing a beam of primary electrons, a frustro-conical target having its apical portion directed toward the cathode and having a central opening in such portion, the said target being conditioned to emit secondary electrons upon. the impingement of primary electrons thereon, a collecting electrode. positioned behind the said target'for receiving primary electrons which pass through the said central opening in the target, focus-contro-lling means positioned between the electron source and the said target and effec- I focus-controlling means, thereby to vary the relative proportions of the primary electron beam which respectively impinge on the'said target and pass through its said ,central opening, and an anode positioned adjacent to the target for receiving a variable number of secondary electrons emitted by the target'in accordance with variationsin the proportion. of the primary beam current impinging thereon. H

' ,WILLI STATZ. FRIEDRICH MALSCH. 

